This invention relates to the pulse propagation time measurement through receivers and, more particularly, to such measurements through the receivers of distance measurement equipment (DME).
There is a growing need in the aviation industry for precision landing guidance systems. Precision landing guidance is typically derived using microwave scanning antennas to provide the angular guidance in conjunction with a precision DME. An example of this is the Microwave Scanning Beam Landing System (MSBLS) built for the Space Shuttle.
There are currently in operation a number of DME systems, the most common of which is referred to as TACAN. In order for TACAN to fill the need of providing precision DME data, its accuracy would have to be increased from the typical .+-.150 feet to .+-.10 feet. A critical component needed to achieve this accuracy is an automatic calibration unit which would periodically measure range errors in the form of receiver and cable delays. This unit would have to function without modification to existing hardware and also without interfering with the normal TACAN equipment operation. The problem is further complicated by the fact that the TACAN equipment can be operating on any one of 252 different frequencies.
The airborne portion of TACAN functions by transmitting an interrogation signal to a ground based TACAN station which responds by transmitting a pulse pair coded signal to the airborne receiver portion of the TACAN equipment. The accuracy to which the airborne TACAN equipment can determine distance to the ground based TACAN station is dependent on the pulse delay through the airborne TACAN receiver on any one of the 252 frequencies it is operating on.
Current pulse delay calibration systems usually consist of laboratory type equipment such as synthesizer type frequency generators, pulse generating equipment, pulse modulation equipment and high speed oscilloscopes, none of which is suitable for airborne use by virtue of it size, weight and lack of ruggedizaton to accommodate the airborne environment.
Proposals have been made to modify existing airborne TACAN receivers to provide the pulse delay measurement function. Modification has been considered because it would be possible to determine the operating frequency of the receiver either mechanically or electrically by direct connection to the internal components of the receiver. Unfortunately, this would require removal and rework of installed and fully operational systems, a costly and unattractive solution.